Find a better way to prevent flicker on enter element fullscreen

[Jeremy Jones]

element fullscreen swapping wkebview and the place holder view and changing the style of the WKWebView is pretty bad.

        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(0 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
            _repaintCallback = VoidCallback::create([self](WebKit::CallbackBase::Error) {
                dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(0 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
                    [self _manager]->willEnterFullScreen();
                });
            });
            _page->forceRepaint(_repaintCallback.copyRef());
        });

This is currently required to make sure web view is completely laid out before entering fullscreen.